1. Field of the Invention
The invention relates to a method for patterning a substrate. In particular, the invention relates to a method for double patterning a substrate using dual-tone development.
2. Description of Related Art
In material processing methodologies, such as those used in the fabrication of micro-electronic devices, pattern etching is often utilized to define the intricate patterns associated with various integrated circuit elements. Pattern etching comprises applying a patterned layer of radiation-sensitive material, such as photo-resist, to a thin film on an upper surface of a substrate, and transferring the pattern formed in the layer of radiation-sensitive material to the underlying thin film by etching.
The patterning of the radiation-sensitive material generally involves coating an upper surface of the substrate with a thin film of radiation-sensitive material and then exposing the thin film of radiation-sensitive material to a pattern of radiation by projecting radiation from a radiation source through a mask using, for example, a photolithography system. Thereafter, a developing process is performed, during which the removal of the irradiated regions of the radiation-sensitive material occurs (as in the case of positive-tone photo-resist), or the removal of non-irradiated regions occurs (as in the case of negative-tone photo-resist). The remaining radiation-sensitive material exposes the underlying substrate surface in a pattern that is ready to be etched into the surface.
As an example, for positive-tone pattern development, a typical lithographic patterning technique is shown in FIGS. 1A and 1B. As shown in FIG. 1A, a layer of positive-tone photo-resist 102 is formed on a substrate 101. The layer of photo-resist 102 is exposed to electromagnetic (EM) radiation 107 through a mask 103. Mask 103 includes transparent portions 104 and opaque features 108 that form a pattern, as shown in FIG. 1A. A distance (or pitch) 109 between opaque features 108 is shown in FIG. 1A. The transparent portions 104 transmit EM radiation 107 to the layer of positive-tone photo-resist 102, and the opaque features 108 prevent EM radiation 107 from being transmitted to the layer positive-tone photo-resist 102. FIG. 1A shows the layer of positive-tone photo-resist 102 having exposed portions 105 that are exposed to EM radiation 107 and unexposed portions 106 that are not exposed to EM radiation 107. As shown in FIG. 1A, mask features 108 are imaged onto the layer of positive-tone photo-resist 102 to produce corresponding photo-resist features aligned with unexposed portions 106.
As shown in FIG. 1B, after removing exposed portions 105 of the layer of positive-tone photo-resist 102, unexposed portions 106 remain on substrate 101 and form the pattern transferred from mask 103 to substrate 101. As shown in FIGS. 1A and 1B, mask features 108 are imaged onto the layer of positive-tone photo-resist 102 to produce corresponding photo-resist features (i.e., unexposed portions 106). As shown in FIGS. 1A and 1B, pitch 110 between unexposed portions 106 is determined by pitch 109 between features 108 of mask 103.
As another example, for negative-tone pattern development, a typical lithographic patterning technique is shown in FIGS. 2A and 2B. As shown in FIG. 2A, a layer of negative-tone photo-resist 202 is formed on a substrate 201. The layer of negative-tone photo-resist 202 is exposed to the EM radiation 207 through a mask 203. The mask 203 includes transparent features 204 that form a pattern and opaque portions 208, as shown in FIG. 2A. A distance (pitch) 209 between transparent features 204 is shown in FIG. 2A. Transparent features 204 transmit EM radiation 207 to the layer of negative-tone photo-resist 202, and opaque portions 208 prevent EM radiation 207 from being transmitted to the layer of negative-tone photo-resist 202. FIG. 2A shows the layer of negative-tone photo-resist 202 having exposed portions 205 that are exposed to EM radiation 207 and unexposed portions 206 that are not exposed to EM radiation 207. As shown in FIG. 1A, mask features 204 are imaged onto the layer of negative-tone photo-resist 202 to produce corresponding photo-resist features aligned with exposed portions 205.
As shown in FIG. 2B, after removing unexposed portions 206 of the layer of negative-tone photo-resist 202, exposed portions 205 remain on substrate 201 and form a pattern transferred from mask 203 to substrate 201. As shown in FIGS. 2A and 2B, mask features 204 are imaged onto the layer of negative-tone photo-resist 202 to produce corresponding photo-resist features (i.e., exposed portions 205). Pitch 210 between exposed portions 205 is determined by pitch 209 between features 204 of mask 203, as shown in FIGS. 2A and 2B.
Photolithographic systems for performing the above-described material processing methodologies have become a mainstay of semiconductor device patterning for the last three decades, and are expected to continue in that role down to 32 nm resolution, and less. Typically, in both positive-tone and negative-tone pattern development, the minimum distance (i.e., pitch) between the center of features of patterns transferred from the mask to the substrate by a lithography system defines the patterning resolution.
As indicated above, the patterning resolution (ro) of a photolithographic system determines the minimum size of devices that can be made using the system. Having a given lithographic constant k1, the resolution is given by the equationro=k1λ/NA,  (1)
where λ is the operational wavelength of the EM radiation, and NA is the numerical aperture given by the equationNA=n·sin θo.  (2)
Angle θo is the angular semi-aperture of the photo-lithography system, and n is the index of refraction of the material filling the space between the system and the substrate to be patterned.
Following equation (1), conventional methods of resolution improvement have lead to three trends in photolithographic technology: (1) reduction in wavelength λ from mercury g-line (436 nm) to the 193 nm excimer laser, and further to 157 nm and the still developing extreme-ultraviolet (EUV) wavelengths; (2) implementation of resolution enhancement techniques (RETs) such as phase-shifting masks, and off-axis illumination that have lead to a reduction in the lithographic constant k1 from approximately a value of 0.6 to values approaching 0.25; and (3) increases in the numerical aperture (NA) via improvements in optical designs, manufacturing techniques, and metrology. These latter improvements have created increases in NA from approximately 0.35 to values greater than 1.35.
Immersion lithography provides another possibility for increasing the NA of an optical system, such as a lithographic system. In immersion lithography, a substrate is immersed in a high-index of refraction fluid (also referred to as an immersion medium), such that the space between a final optical element and the substrate is filled with a high-index fluid (i.e., n>1). Accordingly, immersion provides the possibility of increasing resolution by increasing the NA (see equations (1), and (2)).
However, many of these approaches, including EUV lithography, RET lithography, and immersion lithography, have added considerable cost and complexity to lithography equipment. Furthermore, many of these approaches continue to face challenges in integration and challenges in extending their resolution limits to finer design nodes.
Therefore, another trend in photolithographic technology is to utilize a double patterning approach, which has been introduced to allow the patterning of smaller features at a smaller pitch than what is currently possible with standard lithographic techniques. One approach to reduce the feature size is to use standard lithographic pattern and etch techniques on the same substrate twice, thereby forming larger patterns spaced closely together to achieve a smaller feature size than would be possible by single exposure. During double patterning, a layer of radiation-sensitive material on the substrate is exposed to a first pattern, the first pattern is developed in the radiation-sensitive material, the first pattern formed in the radiation-sensitive material is transferred to an underlying layer using an etching process, and then this series of steps is repeated for a second pattern, while shifting the second pattern relative to the first pattern. Herein, the double patterning approach may require an excessive number of steps, including exiting the coating/developing tool and re-application of a second layer of radiation-sensitive material.
Another approach to double the resolution of a lithographic pattern is to utilize a dual-tone development approach, wherein a layer of radiation-sensitive material on the substrate is exposed to a pattern of radiation, and then a double pattern is developed into the layer of radiation-sensitive material by performing a positive-tone development and a negative-tone development. However, current dual-tone development approaches lack the ability to adjust, control and/or optimize the double pattern formed on the substrate.